Grain-oriented magnetic steel sheets are used as a material for iron core in transformers and other electrical machinery and apparatus and required to have a high magnetic flux density and a low iron loss. These magnetic properties are attained by providing secondary recrystallized structure with a texture having {110} face in parallel to a rolling face and &lt;001&gt; axis along a rolling direction or having so-called Goss orientation as a main direction.
For this purpose, various components including silicon are added to the grain-oriented magnetic silicon steel sheet. However, it is known that the workability lowers and particularly surface cracks and surface defects are apt to be considerably produced through hot rolling. If the degree of the surface defects is conspicuous, not only the appearance is poor, but also the degradation of the properties such as lowering of lamination factor, lowering of interlaminar insulation property and the like is caused. Therefore, it is an important matter how to prevent such surface cracks and surface defects in view of the production step.
As a method of decreasing cracks at the hot rolling step for the grain-oriented silicon steel sheet, there have hitherto been proposed a method of controlling intergranular oxidation by the addition of Mo or the like as described in JP-A-61-9521, a method of decreasing cracks by refining the structure through recrystallization as described in JP-A-2-182832, JP-A-3-115526 and JP-A-62-149815, and the like. However, these methods are not involved in drastic settlements.
Furthermore, JP-A-63-295044 proposes a method of controlling generation of slag by setting an existing time in a high-temperature furnace during the heating of slab to a certain upper limit, which brings about the restriction of operation to lower the productivity.
As mentioned above, the conventional techniques for preventing cracks of silicon steel sheet in the hot rolling do not yet provide satisfactory results.